I not sure what you mean by casting an eggshell - a complete shell
or a fragment. You certainly could not invest eggshell and burn it
our because it consists largely of calcium carbonate rather like sea
shells. I have made castings of sea shells but to do so I have made a
rubber mould using RTV and using a wax injector to make the patterns
for investment,. I could do a casting of a whole egg the same way but
It would need a lot of metal to cast it. Broken fragments might be
mouldable and injectable but it would be a challenge technically.
Artistically I’m not sure what it would do for me.

But some of my college students in my class have actually casted
sections of them/not the whole egg–some successfully and some not.
There is a lot of calcium–we burned out for a long time/maybe 10-12
hours. It was hit or miss. We’ve done snake vertebrae, dried
shrimps, bat wings rat skulls, seahorses and some shell material.
Make sure you spue well and try to gently blow up the main sprue
hole to dislodge any loose ash.–meaning we used a small air
compressor. We cast centrifically and quench after 10 minutes on the
small flasks.

Jo-Ann wrote that, this is John with something else. The calcium is
the least of your worries. If you sprue up an egg, you are going to
get a solid egg if it’s successful. Like five ounces solid. To get a
hollow egg you’d need to do hollow core casting things and that will
be really tricky with an eggshell. You need to fill the shell with
investment AND use a method to hold it in place during burnout or it
will justfall when the shell burns away. IOW, you’d need to poke
holes in the sides to get investment that ties the interior with the
exterior and theregoes your egg. Or saw it in half, cast each half
and solder together after. Easier to just raise it out of sheet,
probably. John D

By the way, last night a student who should have casted last week
and didn’t show up due to work conflict–was able to cast again.
Howe I took out herflask originally and held it for a week in a
second cool kiln. After dropping the temperature for casting & doing
another flask first, we warmed up her flask for an hour at about 800
degreesand it casted just fine…*

And as Mr. J. Donivan (HI!! you 2) stated, the investment internally
has to tie into the investment externally. On bigger pieces (bronze
sculptures, we just make a hole thru the wax and our investment
(ceramic shell) just makes a connection thru these holes. For smaller
pieces that are plaster invested, we use pieces of wire of the same
alloy we are going to be casting in, and push pieces of wire thru the
wax and the internal investment and external investment attach to the
wires so when the wax is removed, the core/internal unit of
investment is held in place. This wire method only works if the
burnout temp that is going to be used is low enough so the metal in
the wire does not melt. If for some reason a high temp burnout is a
must, we use nichrome wire for the pins and later weld the holes
shut. I personally like this latter method with any casting like this
as many times the “high value alloy” wires (gold or silver or ?) just
don’t incorporate into the piece and one has to weld/solder fill the
holes closed anyway.

Also, making an extra “air vent” to the top of the investment where
air can be blown into the investment cavity to assist in removing
calcium “dust” car do a lot in the way of removing this solid
"contaminant" before casting and/or when spruing the item, adding an
extra “catch basin” or reservoir on the “out gas” side of the item
to be cast, to create an area for the ash to be pushed ahead of the
metal entering the mold/void and getting pushed into the void and
out of the piece.

Or make the mold of the egg, and make a hollow wax by sloshing hot
wax in the mold, , it does not have to be solid unless you are
working with very small eggs. We make “sloshed” waxes all the time
for our bronze castings.

I was wondering whether using the lost wax casting method would be
successful way to cast an eggshell?

Actually, I think that most people in this forum know how to do it.
You make a rubber mold of the eggshell. Use that mold to make a
duplicate of the eggshell in wax. Finally, use the lost wax process
to make an eggshell ofprecious metal.

I am certain that the person who said that you can’t use lost wax to
make an egg shell knew this, I just want to keep the record
straight.

Yes to most of a degree you are correct, but the thickness (actually
thinness) of an egg shell is usually too thin to cast directly (mold
to wax to casting) so some form of thickening of the shell before
molding might be helpful in getting consistent, cast-able waxes from
the mold.